|Republican vote in counties of 20,000 or more from 2004|
to 2016; click on the image to enlarge it. (Penn State maps)
"People moving from moderate partisan counties are just as likely to move to extreme partisan counties as they are to move to other moderate counties. However, people who live in a politically extreme county are significantly likely to move to a similarly extreme county," according to a Penn State press release. "This type of political sorting might turn extreme counties into 'magnets' that pull people from moderate counties and exchange them with other extreme counties."